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From: Dennis Zhou <dennis@kernel.org>
To: David Sterba <dsterba@suse.cz>
Cc: Dennis Zhou <dennis@kernel.org>, David Sterba <dsterba@suse.com>,
	Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
	kernel-team@fb.com, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH v2 2/2] btrfs: fix compressed write bio attribution
Date: Fri, 13 Dec 2019 14:21:49 -0800
Message-ID: <20191213222149.GA46346@dennisz-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <20191213122401.GV3929@suse.cz>

On Fri, Dec 13, 2019 at 01:24:01PM +0100, David Sterba wrote:
> On Thu, Dec 12, 2019 at 10:19:34AM -0800, Dennis Zhou wrote:
> > From a0569aebde08e31e994c92d0b70befb84f7f5563 Mon Sep 17 00:00:00 2001
> > From: Dennis Zhou <dennis@kernel.org>
> > Date: Wed, 11 Dec 2019 15:20:15 -0800
> > 
> > Bio attribution is handled at bio_set_dev() as once we have a device, we
> > have a corresponding request_queue and then can derive the current css.
> > In special cases, we want to attribute to bio to someone else. This can
> > be done by calling bio_associate_blkg_from_css() or
> > kthread_associate_blkcg() depending on the scenario. Btrfs does this for
> > compressed writeback as they are handled by kworkers, so the latter can
> > be done here.
> > 
> > Commit 1a41802701ec ("btrfs: drop bio_set_dev where not needed") removes
> > early bio_set_dev() calls prior to submit_stripe_bio(). This breaks the
> > above assumption that we'll have a request_queue when we are doing
> > association. To fix this, switch to using kthread_associate_blkcg().
> 
> Can be kthread_associate_blkcg used also for submit_extent_page that
> calls bio_associate_blkg_from_css indirectly when initializing wbc?
> 
> 2996                 bio_set_dev(bio, bdev);
> 2997                 wbc_init_bio(wbc, bio);
> 2998                 wbc_account_cgroup_owner(wbc, page, page_size);
> 
> wbc_init_bio:
> 
> 	if (wbc)
> 		bio_associate_blkg_from_css();

Correct me if I'm wrong, but I don't think submit_extent_page() is only
called from kthread contexts. So, we wouldn't be able to rely on
kthread_associate_blkcg().

I can think about how to make wbc better for association in general, but
it's a percpu decrement and increment so it shouldn't really be much in
overhead.

Thanks,
Dennis

  reply index

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-12  0:07 [PATCH 1/2] btrfs: punt all bios created in btrfs_submit_compressed_write() Dennis Zhou
2019-12-12  0:07 ` [PATCH 2/2] btrfs: fix compressed write bio attribution Dennis Zhou
2019-12-12 15:18   ` David Sterba
2019-12-12 16:19     ` Dennis Zhou
2019-12-12 18:19   ` [PATCH v2 " Dennis Zhou
2019-12-13 12:24     ` David Sterba
2019-12-13 22:21       ` Dennis Zhou [this message]
2019-12-17 15:05         ` David Sterba
2019-12-17 18:44           ` Dennis Zhou
2019-12-12  0:15 ` [PATCH 1/2] btrfs: punt all bios created in btrfs_submit_compressed_write() Chris Mason
2019-12-30 15:08 ` David Sterba

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